Game designer Soren Johnson (Developer/Designer for Maxis) has posted an interesting article on his blog. The article lists what he considers 8 common mistakes of game design. The comments seem to relate most directly to RTS games but I think they would apply to some degree to all games including the RPG and MMO genre's.
In addition to this article there are plenty of other interesting things to read there.
A couple of points he made that resonated strongly with me were:
1. Hard-core game conventions
One of the most common pitfalls for a game designer is to fear that the game is not hard enough. This fear often leads to hard-core game conventions, like restrictive save systems and unlockable content, that only put roadblocks in the way of the mainstream gamer who is just looking to have a good time.
Fun Factor = Interesting Decisions / Actual Time Played.